1. Field of the Invention
The invention relates to a process for preparing pyrethroid acids from conjugated dienes. In particular, it relates to a processs for producing chrysanthemic acid and its analogs in high yields.
2. Description of the Prior Art
Chrysanthemic acid and its analogs known generically as pyrethroid acids, have long been acknowledged as the effective components of pyrethroid esters, which esters are potent insecticides. It has been proposed in U.S. Pat. No. 4,028,418 to synthesize mixtures of cis- and trans-chrysanthemummonocarboxylic acid and derivatives and other substituted cyclopropanecarboxylic acids by essentially a two step process. In the first step a conjugated diene or the like is reacted with a monohaloketene, which is formed in situ, to produce a mixture of two different stereoisomers of a monohalo-cyclobutanone.
For example, 2,5-dimethyl-2,4-hexadiene is said to react stereospecifically with monochloroketene to give trans-2-chloro-4,4-dimethyl-3-(2'-methyl-1'-propenyl)cyclobutanone and trans-2-chloro-3,3-dimethyl-4-(2'-methyl-1'-propenyl)cyclobutanone. The 2-halo-3,3-dimethyl-and 2-halo-4,4-dimethyl-cyclobutanones, after isolation and purification, are said to be subjected to a ring-contraction step, a Favorskii reaction, by heating them in the presence of a base. The resulting products are a mixture of salts of cis-and trans-chrysanthemummonocarboxylic acid (2,2-dimethyl-3-isobutenylcyclopropane carboxylic acid). The overall yield for that reaction is said to 42% for the first step and 25% for the second step, thus providing a total yield (0.42.times.0.25) of only 10.5% of theoretical.
The cycloaddition of dichloroketene with 2,5-dimethyl-2,4-hexadiene which provided two regioisomers, the beta-vinyl isomer,2,2-dichloro-4,4-dimethyl-3-(2-methylpropenyl)-cyclobutanone and a minor amount of the alpha-vinyl isomer, 2,2-dichloro-3,3-dimethyl-4-(2-methylpropenyl)cyclobutanone, was reported in J. Org. Chem., Vol. 44, No. 1, p. 107-110, (1979). Attempts to reduce certain 2,2-dichlorocyclobutanones disclosed therein with dehalogenating agents, however, yielded primarily dehalo-cyclobutanones and only small yields of monohalo-cyclobutanones.
It has also been reported that certain mono-olefins can provide better yields of dichlorocyclobutanones, when phosphorous oxychloride is added to a reaction mixture containing such a mono olefin and in which dichloroketene is generated therein in situ, J. Org. Chem., Vol. 43, No. 14, p. 2879-2882.